Make Fl_Window::flush() public for consistency with subclasses

... as discussed on fltk.general recently

Note: this method was already public in all subclasses of Fl_Window.

FL/Fl_Window.H

@@ -98,9 +98,14 @@ protected:
   /** Stores the last window that was made current. See current() const */
   static Fl_Window *current_;
   void draw() FL_OVERRIDE;
+
+public:
+
   /** Forces the window to be drawn, this window is also made current and calls draw(). */
   virtual void flush();
 
+protected:
+
   /**
     Sets an internal flag that tells FLTK and the window manager to
     honor position requests.